The conventional detection of various chemical species customarily uses electrochemical sensors specially designed, e.g., for detecting the presence of the chemical species, or for determining the chemical species' concentration in a solution. Such determinations may be based on the property that, within certain limits, the potential of the electrode of the sensing device can be correlated with the chemical species' activity in the solution. The membrane of the electrochemical sensing device is selected so that the electrochemical sensors selectively measures mainly the chemical species of interest. Electrochemical sensors have many applications in the fields of medicine, engineering, industrial processing control, education, and research.
Semiconductor elements, and membrane sensors, in particular, as well as methods for producing membrane sensors on the base of semiconductor substrates such as silicon wafers are already known. Ion selective electrodes (ISE) known in the art usually comprise three common components: the internal reference electrode (typically a silver/silver chloride (Ag/AgCl)); an internal fill solution; and an organic ion selective membrane. In a conventional macro-sized ISE, a Ag/AgCl wire is used as the internal reference electrode, the internal fill solution is contained in a cylindrical plastic tube, commonly called the electrode body, an insulating barrier is provided between the internal fill solution and the sample, and the organic ion selective membrane is affixed to the end of the cylindrical tube. Importantly, the organic membrane must be affixed to form a water tight seal with the cylindrical tube, so that the electrical path is through the organic ion selective membrane. The organic membrane has to form a water tight seal with the electrode body, otherwise the voltage of the electrode is not properly measured. A water tight seal in a macro-sized ISE can be accomplished by bonding the organic membrane to the cylindrical tube, or mechanically compressing the organic membrane to the cylindrical tube.
Efforts have been made to miniaturize electrochemical sensors, and such sensors can have sensing elements with dimensions on the order of microns to less than one micron, for example. However, as ion-selective sensors are made smaller, the surface area to which the ion-selective membrane of the device adheres becomes reduced. A problem in the use of silicon nitride, or other suitable nitrides, as an insulating layer in silicon- or silicon-oxide-based electrochemical sensing devices is the difficulty in adhering encapsulants and ion-selective membranes (coatings) to the nitride. In the fabrication of silicon-based electrochemical sensing devices, electrical leads are typically patterned on a silicon oxide substrate or a silicon oxide layer on top of a silicon substrate. It is generally known that silicon oxide tends to absorb moisture and hydrate, which compromises its insulating properties. Thus, the silicon oxide is usually conformally coated with a water impermeable and electrically insulating layer, such as silicon nitride, to prevent degradation of the silicon oxide. We have found that silicon nitride is a poor surface for adhesion of ion-selective membranes in electrochemical sensing devices. The strength of the adhesion between the membrane and the nitride layer of the sensor structure can become a limiting factor in the useful life of the sensor. If the membrane adhesion is compromised, then ionic species can leak into or out of the area of the membrane that covers the reference electrode, and as a result change the voltage of the reference electrode. Furthermore, the selectivity of the signal across the membrane for the chemical species can be lost if the membrane adhesion is compromised. Therefore, the membrane should completely cover the entire active region of the electrode in order for the sensor measurements to be reliable.
The electrochemical sensing devices described herein can provide a longer useful life by virtue of increasing the adhesion strength between the membrane and the underlying layered structure of the body of the sensor.